System-level considerations for EVSE design

Cost, efficiency, dependability and bidirectionality are four system-level considerations affecting the design of EV charging stations

Electric vehicles (EVs) continue to grow in popularity. To accelerate the EV revolution, the charging infrastructure is scaling up in line with growing demand. Facilitating the switch to low-carbon transportation is taking effort.

Electric vehicle supply equipment (EVSE) and charging stations are crucial components of the EV ecosystem. Several elements must be considered to ensure that home, public and commercial charging equipment is effective, accessible and user friendly. System engineers should consider several factors before designing EV supply equipment for any charging station.

Basic requirements for the charging station

To get the optimum return over the EVSE’s lifetime, it is critical to evaluate the fundamental requirements of any specific charging station based on its purpose and location. These requirements may not always be the same. When considering the high-level design of a charging station, look at how it will be used and where it is installed. These high-level considerations will impact the low-level electrical design.

These considerations will be influenced by whether the charging station is intended for public use in a high- or low-traffic area, or for residential use. The location will have a substantial impact on functionality and, subsequently, design.

At the planning phase, determining the charging station’s maximum available input power and the required output power is important. Both directly influence the charging speed, as will the design topology chosen based on these parameters. Higher output requires more input power but allows for faster charging, reducing the time users need to spend at the charging station.

But fast charging may not be possible or inappropriate for the location. A more efficient conversion topology reduces the demand on input power, allowing higher charge station density in one location. But this requirement may also impact other factors, such as unit cost or reliability. These considerations are more relevant for public stations, where users are likely to require quick, on-the-go charging.

The system-level factors

OEMs must establish the relative priority of four key factors. These factors are cost, efficiency, reliability and functionality. Each has a direct impact on the topology used. It may not be feasible to optimize a design for all four factors, so prioritization will help focus resources. On a wider basis, an industry-wide approach that balances these variables will result in a charging infrastructure that is not only financially viable but also capable of addressing the needs of the electric vehicle ecosystem.

UNIT COST

Large volume deployment is a crucial factor in the design of electric vehicle charging stations, as it directly affects accessibility and scalability of the charging network. The topologies used, as well as the choice of charging hardware, energy storage systems and network connectivity, are significant contributors to the overall cost. Additionally, factors like site selection, availability of renewable energy sources and grid connection fees should be carefully evaluated to optimize the cost-effectiveness of each charging station.

EFFICIENCY

To maximize the available energy and minimize charging times, efficiency will be the highest priority for some EV charging stations. Charging efficiency goals will influence the circuit topologies used, the power electronics (IGBTs or SiC/GaN power devices), and the integration of smart charging algorithms. OEMs prioritizing efficiency must focus on optimizing the power conversion processes, minimizing losses, and incorporating smart features that enable dynamic adjustments based on grid conditions and user demands.

RELIABILITY

Time between failure could be a deciding factor for the efficacy of a charging station. Design considerations for reliability encompass the robustness of hardware, fault tolerance, and the implementation of redundant systems. The selection of high-quality components, weather-resistant materials, proper thermal management, and advanced monitoring and maintenance systems are essential to ensure uninterrupted service. For public charging stations, designers need also to consider factors such as cybersecurity to protect against potential threats and ensure the secure and reliable operation of the charging station.

FUNCTIONALITY

One emerging area to consider is bidirectional charging. More consumers are learning about vehicle-to-grid (V2G) capabilities, enabling EVs to not only consume energy but also return energy to the grid during peak demand periods. This bidirectional flow of energy requires advanced power electronics (i.e. specific types of power discrete, power modules and topologies), grid communication protocols, and smart charging algorithms. Bidirectionality has the potential to transform EVs into distributed energy resources during their idle time, contributing to grid stability and resilience. Currently, bidirectional charging is aimed at domestic chargers, but that may change in the future.

Basic components of a charging station

The rate of charge will depend on the use case as well as the available power. In a domestic environment, the EV can be left plugged in for a long period–e.g. overnight–and therefore a slower single-phase charger will be deployed. Roadside chargers, for example at service stations, must prioritize convenience. This points to rapid charging and a high availability of chargers to meet peak demand. In this scenario, three-phase chargers will be commissioned.

The typical block diagram of an EV fast-charging station is shown in Table 1. The main components of this hardware architecture, which can deliver an output power ranging from 50kW up to 350kW, are:

• EMI filtering: This block filters the three-phase AC input power, reducing the electromagnetic radiation generated by the charger.
• PFC: The Power Factor Correction (PFC) stage ensures that the power drawn from the electrical grid is used more efficiently by aligning the current and voltage waveforms, reducing reactive power and minimizing energy losses.
• AC/DC converter:This converts alternating current (AC) from the power grid to the direct current (DC) required to charge the electric vehicle’s battery. If the charger is supplied from a DC power source, this block is not required.
• DC/DC converter:This block is responsible for adjusting the voltage level of the direct current (DC) power converted from AC to match the electric vehicle’s battery (up to 800V batteries are available today), ensuring efficient power transfer from the charging stating on to the vehicle.
• EV charger control:The charge controller manages the flow of electricity between the power grid and the electric vehicle. It regulates the charging process to ensure optimal performance, prevent overcharging and protect the battery. As shown in Table 1, it supplies the required PWM signal to the gate drivers connected to the switching power converters.
• Metrology block:This block is tasked with the precise measurement and monitoring of the relevant electrical parameters while the charging procedure is in progress. It is particularly important for public chargers, where it must accurately measure and quantify energy consumption, voltage, current and other pertinent parameters to generate precise data for invoicing. In all applications, metrology is required to analyze system performance and guarantee the charging station’s overall dependability and efficiency.

Miscellaneous components include connectors, cables, relays and actuators.

The main types of topologies used for the AC/DC and DC/DC converters, each with its advantages and disadvantages, are summarized below:

Table 1: Most commonly used topologies for AC/DC rectifiers (Source: Avnet)

Table 2: Most commonly used topologies for DC/DC converters (Source: Avnet)

Grid interface

In addition to the system-level considerations, the OEM’s customer will also be looking at the requirements from the point of view of the existing energy infrastructure. There is a complex relationship between charging stations and the power grid. The effect on the grid of one charging station’s power demand must be compared to the demand of many. The user experience is inextricably linked to the amount of electricity the local grid can deliver.

Demand for output power could place strain on the grid during busier periods. The EVSE converter design topology must consider vulnerabilities caused by supply fluctuations. Meeting the escalating need for power from electric vehicle drivers and preserving the electrical grid’s stability requires collaboration.

Integrating photovoltaics (PV) and other forms of renewable energy sources into charge station infrastructure improves sustainability while reducing reliance on conventional power sources. Renewable sources, however, are inherently less dependable. PV panels do not generate power at night. As renewables are less reliable, power from the grid must be available as a backup. If demand from the connected EVs exceeds the power available, output power–and therefore charging speed–must be adjusted. Some converter topologies will be better suited to this scenario than others.

Another trend emerging in this sector is the integration of batteries as a buffer between the grid and the EVSE. Many OEMs are already using batteries for intermediary energy storage. A battery achieves two objectives. Local energy storage reduces instantaneous demand on the grid, it also allows fast chargers to be installed in areas where 3-phase supplies are not already present.

Conclusion

Charger type influences design decisions.

• Residential L2 AC:Unit cost is the main consideration.
• Residential L2 DC: Functionality, including bidirectional charging, drives decisions.
• Commercial L2 Destination Charger:For charging-as-a-service operators, unit cost has given way to efficiency.
• Commercial L3 DC Fast Charging:Reliability and functionality (compatibility) are becoming key drivers. Some countries are providing subsidies to accelerate this part of the EV charging infrastructure.

Cost, efficiency, dependability and bidirectionality are four system-level considerations affecting the design of EV charging stations. The key to a well-designed charging station is an appreciation and prioritization of these aspects. This will ensure a dependable, efficient and cost-effective infrastructure that can handle the increasing demand for electric vehicles and help the power grid remain resilient and sustainable.